There's a couple ways:

- Signal could just ban all UK IP addresses from accessing its messaging servers. Your encrypted messages still pass through servers controlled by Signal, Inc.

- UK government could require ISPs to deny access to Signal's messaging servers.

Of course, one could just use a VPN, set up a VM in a third country and run a Signal Proxy Server, etc.

It was hard enough getting family/friends to download/use Signal, as its UX is sub-par compared to alternatives.

If you start telling them they have to use a VPN, or run a server, etc, that will kill the network effect in UK.